1. Field of the Invention
The present invention relates to a nitride semiconductor. More particularly, the present invention relates to a fabrication method of nitride semiconductors, by which a sapphire substrate is surface-reformed to grow a nitride semiconductor layer on the sapphire substrate via Metal-Organic Chemical Vapor Deposition (MOCVD) without formation of a low temperature buffer layer, and a nitride semiconductor fabricated thereby.
2. Description of the Related Art
A Light Emitting Diode (LED) as a kind of optoelectric device basically consists of a junction of p- and a n-doped semiconductor layers. When applied with electric current, the LED converts energy corresponding to its band gap into light through electron-hole combination. Recently, full color LEDs are produced based upon GaN compounds to emit blue, green and ultraviolet lights.
A GaN LED has a drawback that a GaN layer grown directly on a sapphire (Al2O3) substrate forms an opaque layer therebetween because GaN poorly wets with sapphire owing to the lattice constant mismatch.
A solution to this problem is disclosed in the U.S. Pat. No. 5,290,393, entitled “Crystal Growth Method for Gallium Nitride-Based Compound Semiconductor.” This solution proposes to fabricate a GaN-based semiconductor by growing a buffer layer of nitride compound such as GaN, AlN and InN on a sapphire substrate at a low temperature of about 500° C. and then a GaN layer on the buffer layer at a high temperature.
A low temperature GaN buffer layer grown to a thickness of 20 to 30 nm is polycrystalline or has a partial metastable cubic structure. After growth of the low temperature buffer layer, temperature elevation causes GaN compound to crystallize transforming its phase partially into a stable hexagonal structure and the surface morphology of GaN compound also changes coarse forming islands. As a hot temperature GaN layer is formed on the buffer layer, the islands of the low temperature buffer layer begin to coalesce together thereby forming a fine GaN layer.
However, this solution has a drawback that surface defects such as stacking faults and threading dislocations may occur during coalescence of the islands.
In the meantime, Korean Patent Application Publication No.2000-0055374 and the U.S. Pat. No. 6,528,394 claiming its priority, entitled “Growth Method of Gallium Nitride Film”, propose a GaN layer growth method via Hydride Vapor Phase Epitaxy (HVPE).
According to this document, a fine GaN layer is formed on a sapphire substrate through first nitrification, pre-treatment with ammonia (NH3) and HCl gas, second nitrification and hot GaN film formation. In this circumstance, the term “nitrification” refers to unintentional formation of a thin film for example of AlN on the sapphire by flowing NH3 gas onto the sapphire substrate but not to intentional formation of an AlN buffer layer.
This method is advantageous for growing a relatively thin GaN layer of about 100 to 200 μm thanks to high GaN growth rate of HVPE. However, in LED fabrication requiring a relatively thin GaN layer of about 3 to 5 μm, there is a drawback that it is difficult to control film thickness or ensure film quality.
Furthermore, although HVPE can advantageously form undoped and n-doped GaN layers on a sapphire substrate, it is necessary to load the sapphire substrate again into a Metal-Organic Chemical Vapor Deposition (MOCVD) reactor after the growth of the GaN layer to form an active region and p-doped GaN layers on the sapphire substrate to fabricate an LED.